1. Field of the Invention
The present invention relates in general to sound or noise attenuating materials and structures primarily for use in the control of noise and noise pollution, but also for use as a thermal insulators and for auxiliary uses in the medical and construction fields, and wherever it is necessary to attenuate sound or noise emission or transmission. More particularly, the present invention is directed to an improved noise attenuation panel which includes as a component thereof a compound that can be fabricated, formed, and cured in a variety of forms and shapes to provide relatively thin surfaces which yield substantial noise attenuation. The compound is a composite type consisting of an adhesive binder-base having an elastic inter-molecular structure providing elastic wave attenuating properties and individually encapsulating in an homogeneous mixture a plurality of hollow microspheres having a substantially reduced atmosphere therein.
The present invention also pertains to the field of laminated structures, and in particular to those laminated structures used for insulation both thermal and acoustical.
Noise attenuation as used herein includes both sound absorption and sound transmission loss.
2. Description of the Prior Art
This invention pertains to noise attenuation structures which utilize the acoustical compound disclosed in U.S. Pat. No. 4,079,162 issued to applicant and improved varieties of that compound. In the present invention, that compound is combined with various other elements to form laminated structures which are effective in improving sound absorption and sound transmission loss. Sound absorption is the reduction of reflected noise waves within the area in which a noise source is confined. Sound transmission loss is the reduction of noise passing through a barrier between the noise source and the listener.
Traditional and commercially available structures which perform these functions with a reasonable degree of effectiveness are bulky, heavy structures, the most widely sold at present weighing twenty pounds per square foot and being four inches thick. Using the laminated panel of this invention, the weight can be reduced to six to eight pounds per square foot and the thickness can be reduced to one to two inches.
Noise pollution has become an ever increasing problem within recent years. Because of the increasing interest by environmentalists, as evidenced by the enactment of governmental regulations, there is an increased requirement to protect from and/or restrain sound emission. There are techniques available to achieve sound reduction or confinement, but these techniques have certain limitations or disadvantages associated therewith.
Noise or sound in the audio frequency range is transmitted by the interaction of molecules of gas or mixture of gases such as air. Noise in the audio frequency range may be transmitted through a rigid partition, such as a wall, by forcing the partition into vibration. A vibrating partition becomes a secondary source radiating sound to the side opposite the original source. Over a large portion of the audio frequency range of 128 to 2048 Hz, approximately 4 to 5 db is lost each time the weight of the partition is doubled.
It is further known that a governing material principal is (r).sup.2 +a+(t).sup.2 =1; that is, the total energy impinging on a barrier will be partly reflected (r).sup.2, partly absorbed a, and partly transmitted through the material (t).sup.2. It is also known that each time an elastic wave passes through the surface of one medium to another, and if the densities of the media are different, there will be a refraction of the wave. It is the distinct purpose of the present invention to substantially reduce reflection (r) and to increase absorption (a) substantially over the prior art. Thus rather than reflect and transmit sound waves, the present compound acts to absorb sound waves superior to the prior art.
The majority of the prior art has relied upon one or more of three properties of materials, those being thickness, density and porosity, to achieve varying levels of sound wave attenuation in acoustic materials. The usual process to obtain improved acoustic attenuation is to increase the thickness of a wall or partition. However, there are disadvantages associated with this practice such as the attendant cost increase, weight increase and massive thickness. It has been customary to depend on not only thickness, but also density and porosity to achieve varying levels of elastic wave attenuation in acoustical materials, but the better performing attenuating materials are extremely dependent upon thickness and density.
Accordingly, it is an object of the present invention to provide a noise attenuation material and laminated noise attenuation structure that can provide an effective sound attenuation structure while being relatively thin and light in weight. The present invention uses the principles of two physical phenomena. These are utilized, in combination with the techniques of the prior art, to significantly enhance the inherent sound attenuation characteristics of the acoustical material and laminated structures of the present invention.
The first of such phenomena is that a sound wave has "elastic" properties in its behavior. It is known that an elastic wave will tend to set in motion the molecules of a substance upon which it impinges. The impinged material will tend to move as a direct function of the impinging wave. It is also known that the impinged material will, depending on its resonant characteristics, absorb varying amounts of the energy contained in the sound wave due to this natural phenomenon. Hence, a material having improved sound attenuating characteristics should be a material having a very good low frequency (100 to 2000 Hz) vibration/shock absorption properties. Such a material should be relatively soft, flexible and elastic in its behavior in the presence of acoustical or sound waves.
A second and most obvious phenomenon is the fact that audio frequency sound waves are dependent upon the existence of gaseous molecules for the transmission of sound. A reduction of air pressure or density of the gaseous environment will reduce the efficiency of sound transmission through the medium. Sounds cannot be transmitted through a vacuum. This phenomenon leads to the use of hollow microspheres, having reduced atmospheric pressure therein. The lower the atmospheric pressure within said microspheres, the more efficient the material containing the microspheres will be. Obviously also, the greater the number or volume of hollow microspheres within the material binder, the more efficient the sound attenuating compound will be. Therefore, as the interior atmospheric pressure within the microspheres approaches a vacuum, the sound attenuation material densely packed with said microspheres will reach its maximum efficiency. Also the compound of the present invention is mixed and prepared so as to exclude substantially all free air.
To the inventor's knowledge only one prior art patent, U.S. Pat. No. 3,632,703 Sullivan et al, utilizes an acoustical material substantially similar to that used in the present invention. Its purpose is to provide an acoustical window with a predetermined impedance and density; that is, a material with a velocity of acoustic propagation and density equal to that of sea water.
The key differences between the acoustical compound used in the present invention and Sullivan's disclosure are inter-molecular elasticity of the material, the means and methods of construction and the ultimate results achieved. Sullivan does not appear to be concerned, as is the present invention, specifically with the attenuation of sound. The present invention relies upon and utilizes the difference of the refraction caused by its components, substantially due to the hollow microspheres, whereas Sullivan is concerned only with the velocity of acoustic propagation and its interrelation with density.
The prior art relates the use of polyurethane resins to provide the matching of Sullivan. Sullivan notes that such materials deform easily. Thus his invention simply used a limited number of capsules to decrease the density of a higher density and more rigid resin, combined with a hardener, which were, after preliminary curing, subjected to pressures of 5000-30,000 psi to cause a fracturing of a predetermined number of capsules. This increased the density of the material and causes it to have an acoustic propagation equal to water in a known pressure range. Where the capsules are frangible glass spheres, a great percentage of the capsules are fractured. Sullivan uses ratios of 88% resin base, 6.2% capsules and 5.8% hardener, by weight. The present invention uses one part resin base to 1 to 4 parts microspheres by volume in its acoustical material which is only one layer in the laminated noise attenuation panel.
The microspheres in Sullivan, being substantially uniform in size, range in size from 20 to 80 microns and are uniformly distributed throughout the resin. In the present material they range in size from 10 to 250 microns, and the various sizes are mixed randomly to improve the dispersion of acoustic waves within the material and within the panel if used elsewhere. Also in contrast, the present invention preferably has no spheres in contact with each other in its acoustical compound.
The application of pressure to the material of Sullivan also entraps gas molecules in the partially cured compound when the spheres are fractured to increase the density. In contrast, the fracturing of microspheres or the introduction of a gas or air into the compound material of the present invention is detrimental to its performance. The compound then would not as effectively deter the passage of sound waves through the materials due to the entrapped gas molecules transmitting the wave and due to the loss of a surrounding refractory boundary.
The present invention seeks to minimize reflection from the surface of the noise attenuation panel and compound, to increase absorption of the panel and to minimize transmission of waves to other surfaces. As illustrated in the figures and described in the specification of U.S. Pat. No. 4,079,162, every time a sound wave impinges on the barrier of new material, it is reflected, absorbed and retransmitted. The inventor herein utilizes these phenomena and has developed the novel laminated panel of this disclosure accordingly. In fact, the present disclosure wants the refraction and absorption phenomena to happen an infinite number of times within a high impedance material and other layers in the lamination until, theoretically at least, all the acoustical energy is dissipated into heat energy, whereas Sullivan simply sought a material which would create a low impedance window having a fixed acoustic propagation identical to water. Given these different goals, Sullivan uses an extremely hard and dense compound. The present invention uses as one element in its structure a soft, flexible and less dense compound, its binder having an elastic inter-molecular structure in the cured state in contrast to the rigid inter-molecular structure of Sullivan. The adhesive binder-base material acts as a shock wave absorber. In mechanics, a good shock wave absorber has flexure and resistance to reaction to the impinged mechanical energy. The same is true for acoustics. Once an acoustical wave impinges on the surface of a material, it is the ability of the material to respond as a shock absorber that transfers the mechanical energy of the sound wave into dissipating heat energy.
In contrast, the rigid system of Sullivan provides a highly reflective and transmissive component identical to water with minimal or no absorbative component contemplated.
Thus the present laminated panel exhibits sound attenuating properties heretofore unexpected. The combination of materials yields a structure which substantially reduces reflection of sound from its surface to a minimum and substantially increases refraction and absorption of sound waves within the structure, thus dispersing and transposing the same into heat energy.